Thermite reactions are highly exothermic reactions. During such reactions initially solid reactants undergo oxidation and reduction processes which liberate heat and form reaction products. Such thermite reaction processes serve various useful purposes. Important applications of the thermite reaction process include the welding of metallic members and the cast forming of metal or ceramic parts. In such applications the thermite reaction is utilized to produce a superheated molten metal to cast a part or produce a weld metal for the welding and joining of the members.
Thermite reactions are generally described as reactions between metal oxides and metallic reducing agents. The metal oxides chosen for the reaction are those which have low heats of formation. The reducing agents chosen for the reaction are those which exhibit oxide species with high heats of formation. The difference in the heat of formation of the reaction product metal oxide and the reactant metal oxide is the heat produced in the reaction, and, as indicated, such reaction are highly exothermic. Thermite reactions of particular interest due to their extensive industrial usage are as follows:
______________________________________ Heat Evolved Thermite Reactions K cal ______________________________________ (1) 3Fe.sub.3 O.sub.4 + 8Al = 9Fe + 4Al.sub.2 O.sub.3 719 (2) 3FeO + 2Al = 3Fe + Al.sub.2 O.sub.3 187 (3) Fe.sub.2 O.sub.3 + 2Al = 2Fe + Al.sub.2 O.sub.3 181 (4) 3CuO + 2Al = 3Cu + Al.sub.2 O.sub.3 275 (5) 3Cu.sub.2 O + 2Al = 6Cu + Al.sub.2 O.sub.3 260 ______________________________________
In present commercial form the thermite reactions noted above all require local temperatures of approximately 1750.degree. F. in order to be self-propagating (i.e., in order to ignite and continue the reaction to completion). For this reason, starting materials of lower ignition temperatures (about 850.degree. F.) are placed in direct contact with the thermite reaction materials. Such starting materials may be conveniently ignited with a flint igniter, or other like sparking or ignition device. Upon ignition of the starting material, the starting material serves to ignite the higher temperature ignition point thermite reaction materials.
Thermite reactions are generally conducted in a reaction containment vessel such as a crucible. Upon ignition of the thermite reaction materials significant amounts of hot gases containing smoke and fume evolve from the reaction containment vessel. Such smoke and fume generally includes particulate matter.
When an exothermic reaction occurs, high temperatures are reached in a very short period of time thus causing the surrounding air to expand and flow upwardly out of the vessel or crucible where the reaction takes place. Along with this sudden upward rush of hot gases there is, in addition to flame and spatter, a considerable amount of particulate matter in the form of dust or smoke. Simply to plug the exhaust exits would cause pressure build-up in the vessel or crucible which would then force unreacted exothermic material or slag into the weld chamber or force molten weld metal out of the part receiving holes, or in a multi-part mold, cause separation of the parts, all of which is unacceptable.
Accordingly it is desirable to reduce the amount of particulate emissions, flame and spatter exiting the apparatus during the exothermic process while avoiding excessive pressure build-up.